Coherent LiDAR system utilizing a non-reciprocal optical element

ABSTRACT

A light detection and ranging (LiDAR) system according to the present disclosure comprises an optical circulator and one or more photodetectors (PDs). The optical circulator is to receive an optical beam and transmit the optical beam to a target, to receive a target return signal from the target and to transmit the target return signal to the one or more PDs, where the one or more PDs are to mix the target return signal with an LO signal to generate a heterodyne signal to extract range and velocity information of the target.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.17/105,076 filed on Nov. 25, 2020, the entire contents of which areincorporated herein by reference in their entirety.

TECHNICAL FIELD

The present disclosure relates generally to light detection and ranging(LiDAR) systems, and more particularly to a coherent LiDAR systemutilizing polarization-diverse architecture.

BACKGROUND

Conventional LiDAR systems collect either a single polarization state orlimited polarization states of light scattered or returned from a targetor a target environment. However, since certain diffusive targets mayscatter an incident light with one polarization state into a returnedoptical beam or a returned light signal with multiple arbitrarypolarization states, such systems suffer from an inherent inefficiencyin the amount of light collected from the target, thereby reducing thecoherently mixed heterodyne signal. As such, there is a need to developa LiDAR system with a polarization-diverse architecture that enables thecollection and mixing of all or dominant polarization states scatteredfrom the target in order to improve the efficiency of a heterodynesignal.

SUMMARY

The present disclosure describes various examples of LiDAR systemsutilizing polarization-diverse architecture.

As an example, disclosed herein is a LiDAR system with an opticalsubsystem configured to efficiently collect all possible polarizationstates in a returned target signal scattered from a target andcoherently mixing the returned target signal with an appropriate LOsignal to obtain range and velocity profile of the target, e.g., byusing FMCW LiDAR architecture. A method to collect all possiblepolarization states in the returned target signal scattered from thetarget, and to coherently mix the returned target signal with theappropriate LO signal in a LiDAR system is also disclosed. Differentcombinations of polarization optics, including linear polarizers,wave-retardation optics, and magneto-optics such as Faraday rotators areutilized to achieve the above discussed objectives. The technique may beutilized in both a coaxial system and a non-coaxial system, as well asin both a free-space optics based system and a waveguide (single-mode ormulti-mode waveguide in fiber as well as semiconductor or dielectricphotonic circuit architecture) based system.

In one example, a LiDAR system according to the present disclosureincludes a combination of polarizers, wave plates and magneto-opticalelements by using a polarization-diverse architecture, where allpolarization states in a scattered target signal are collected and mixedwith a local oscillator (LO) signal to generate a coherent heterodynesignal. The LiDAR system improves the collection and mixing efficiencyof the scattered target signal with the LO signal, thereby improving asignal-to-noise ratio (SNR) of the system.

In one example, a LiDAR system according to the present disclosurecomprises an optical source to emit an optical beam and an opticalcirculator to receive the optical beam and transmit the optical beam toa target, to receive a target return signal from the target and totransmit the target return signal to one or more photodetectors (PDs).The optical circulator is configured to collect all polarization statesin the target return signal. The LiDAR system further comprises anoptical element to generate a local oscillator (LO) signal. The LiDARsystem further comprises the one or more PDs to mix the target returnsignal with the LO signal to generate a heterodyne signal to extractrange and velocity information of the target.

In one example, a method in a LiDAR system according to the presentdisclosure comprises emitting an optical beam with an optical source,receiving, by a circulator, the optical beam; transmitting, by thecirculator, the optical beam to a target; receiving, by the circulator,a target return signal returned from the target, including collecting,by the circulator, all polarization states in the target return signal;transmitting, by the circulator, the target return signal to one or morephotodetectors (PDs); generating, by an optical component, a localoscillator (LO) signal; and mixing, by the one or more PDs, the targetreturn signal with the LO signal to generate a heterodyne signal toextract range and velocity information of the target.

These and other aspects of the present disclosure will be apparent froma reading of the following detailed description together with theaccompanying figures, which are briefly described below. The presentdisclosure includes any combination of two, three, four or more featuresor elements set forth in this disclosure, regardless of whether suchfeatures or elements are expressly combined or otherwise recited in aspecific example implementation described herein. This disclosure isintended to be read holistically such that any separable features orelements of the disclosure, in any of its aspects and examples, shouldbe viewed as combinable unless the context of the disclosure clearlydictates otherwise.

It will therefore be appreciated that this Summary is provided merelyfor purposes of summarizing some examples so as to provide a basicunderstanding of some aspects of the disclosure without limiting ornarrowing the scope or spirit of the disclosure in any way. Otherexamples, aspects, and advantages will become apparent from thefollowing detailed description taken in conjunction with theaccompanying figures which illustrate the principles of the describedexamples.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of various examples, reference is nowmade to the following detailed description taken in connection with theaccompanying drawings in which like identifiers correspond to likeelements:

FIG. 1A is a block diagram illustrating an example LiDAR systemaccording to embodiments of the present disclosure.

FIG. 1B is a time-frequency diagram illustrating an example of FMCWLIDAR waveforms according to embodiments of the present disclosure.

FIG. 2 is a block diagram illustrating an example of an opticalsubsystem of a LiDAR system according to embodiments of the presentdisclosure.

FIG. 3 is a block diagram illustrating an example of an opticalsubsystem of a LiDAR system according to embodiments of the presentdisclosure.

FIG. 4 is a block diagram illustrating an example of an opticalsubsystem of a LiDAR system according to embodiments of the presentdisclosure.

FIG. 5 is a block diagram illustrating an example of an opticalsubsystem of a LiDAR system according to embodiments of the presentdisclosure.

FIG. 6 is a block diagram illustrating an example of an opticalsubsystem of a LiDAR system according to embodiments of the presentdisclosure.

FIG. 7 is a block diagram illustrating an example of an opticalsubsystem of a LiDAR system according to embodiments of the presentdisclosure.

FIG. 8 is a flow diagram illustrating an example method in a LIDARsystem according to embodiments of the present disclosure.

DETAILED DESCRIPTION

Various embodiments and aspects of the disclosures will be describedwith reference to details discussed below, and the accompanying drawingswill illustrate the various embodiments. The following description anddrawings are illustrative of the disclosure and are not to be construedas limiting the disclosure. Numerous specific details are described toprovide a thorough understanding of various embodiments of the presentdisclosure. However, in certain instances, well-known or conventionaldetails are not described in order to provide a concise discussion ofembodiments of the present disclosures.

The present disclosure describes examples of coherent LiDAR systems, forexample, frequency-modulated continuous-wave (FMCW) LiDAR systems, andexample methods therein. The described LIDAR system may be implementedin any sensing market, such as, but not limited to, transportation,manufacturing, metrology, medical, and security systems. According tosome embodiments, the described LiDAR system may be implemented as partof a front-end coherent LiDAR system (e.g., a FMCW LiDAR device) thatassists with spatial awareness for automated driver assist systems, orself-driving vehicles.

Range and velocity profile of a target may be measured using a coherentLiDAR system, e.g., a FMCW LiDAR system, wherein backscattered lightfrom the target may be mixed with an LO signal to generate a coherentheterodyne signal, from which range and velocity information of thetarget may be extracted. Maximizing a collection and mixing efficiencyof a scattered target signal with the LO signal is important to increasea signal-to-noise ratio (SNR) of the system, which in turn enhances adetection sensitivity. The target may scatter an incident light intoscattered light with multiple arbitrary polarization states, regardlessof a polarization state of the incident light. By maximizing thecollection and mixing of the scattered target signal with the LO signalin all polarization states, an improvement in the SNR of the system maybe realized.

FIG. 1 illustrates a LiDAR system 100 according to exampleimplementations of the present disclosure. The LiDAR system 100 includesone or more of each of a number of components, but may include fewer oradditional components than shown in FIG. 1. As shown, the LiDAR system100 includes optical circuits 101 implemented on a photonics chip. Theoptical circuits 101 may include a combination of active opticalcomponents and passive optical components. Active optical components maygenerate, amplify, and/or detect optical signals and the like. In someexamples, the active optical component includes optical beams atdifferent wavelengths, and includes one or more optical amplifiers, oneor more optical detectors, or the like.

Free space optics 115 may include one or more optical waveguides tocarry optical signals, and route and manipulate optical signals toappropriate input/output ports of the active optical circuit. The freespace optics 115 may also include one or more optical components such astaps, wavelength division multiplexers (WDM), splitters/combiners,polarization beam splitters (PBS), collimators, couplers or the like. Insome examples, the free space optics 115 may include components totransform the polarization state and direct received polarized light tooptical detectors using a PBS, for example. The free space optics 115may further include a diffractive element to deflect optical beamshaving different frequencies at different angles.

In some examples, the LiDAR system 100 includes an optical scanner 102that includes one or more scanning mirrors that are rotatable along anaxis (e.g., a slow-moving-axis) that is orthogonal or substantiallyorthogonal to the fast-moving-axis of the diffractive element to steeroptical signals to scan a target environment according to a scanningpattern. For instance, the scanning mirrors may be rotatable by one ormore galvanometers. Objects in the target environment may scatter anincident light into a return optical beam or a target return signal. Theoptical scanner 102 also collects the return optical beam or the targetreturn signal, which may be returned to the passive optical circuitcomponent of the optical circuits 101. For example, the return opticalbeam may be directed to an optical detector by a polarization beamsplitter. In addition to the mirrors and galvanometers, the opticalscanner 102 may include components such as a quarter-wave plate, lens,anti-reflective coating window or the like.

To control and support the optical circuits 101 and optical scanner 102,the LiDAR system 100 includes LiDAR control systems 110. The LiDARcontrol systems 110 may include a processing device for the LiDAR system100. In some examples, the processing device may be one or moregeneral-purpose processing devices such as a microprocessor, centralprocessing unit, or the like. More particularly, the processing devicemay be complex instruction set computing (CISC) microprocessor, reducedinstruction set computer (RISC) microprocessor, very long instructionword (VLIW) microprocessor, or processor implementing other instructionsets, or processors implementing a combination of instruction sets. Theprocessing device may also be one or more special-purpose processingdevices such as an application specific integrated circuit (ASIC), afield programmable gate array (FPGA), a digital signal processor (DSP),network processor, or the like.

In some examples, the LiDAR control systems 110 may include a signalprocessing unit 112 such as a digital signal processor (DSP). The LiDARcontrol systems 110 are configured to output digital control signals tocontrol optical drivers 103. In some examples, the digital controlsignals may be converted to analog signals through signal conversionunit 106. For example, the signal conversion unit 106 may include adigital-to-analog converter. The optical drivers 103 may then providedrive signals to active optical components of optical circuits 101 todrive optical sources such as lasers and amplifiers. In some examples,several optical drivers 103 and signal conversion units 106 may beprovided to drive multiple optical sources.

The LiDAR control systems 110 are also configured to output digitalcontrol signals for the optical scanner 102. A motion control system 105may control the galvanometers of the optical scanner 102 based oncontrol signals received from the LIDAR control systems 110. Forexample, a digital-to-analog converter may convert coordinate routinginformation from the LiDAR control systems 110 to signals interpretableby the galvanometers in the optical scanner 102. In some examples, amotion control system 105 may also return information to the LiDARcontrol systems 110 about the position or operation of components of theoptical scanner 102. For example, an analog-to-digital converter may inturn convert information about the galvanometers' position to a signalinterpretable by the LIDAR control systems 110.

The LiDAR control systems 110 are further configured to analyze incomingdigital signals. In this regard, the LiDAR system 100 includes opticalreceivers 104 to measure one or more beams received by optical circuits101. For example, a reference beam receiver may measure the amplitude ofa reference beam from the active optical component, and ananalog-to-digital converter converts signals from the reference receiverto signals interpretable by the LiDAR control systems 110. Targetreceivers measure the optical signal that carries information about therange and velocity of a target in the form of a beat frequency,modulated optical signal. The reflected beam may be mixed with a secondsignal from a local oscillator. The optical receivers 104 may include ahigh-speed analog-to-digital converter to convert signals from thetarget receiver to signals interpretable by the LiDAR control systems110. In some examples, the signals from the optical receivers 104 may besubject to signal conditioning by signal conditioning unit 107 prior toreceipt by the LiDAR control systems 110. For example, the signals fromthe optical receivers 104 may be provided to an operational amplifierfor amplification of the received signals and the amplified signals maybe provided to the LIDAR control systems 110.

In some applications, the LiDAR system 100 may additionally include oneor more imaging devices 108 configured to capture images of theenvironment, a global positioning system 109 configured to provide ageographic location of the system, or other sensor inputs. The LiDARsystem 100 may also include an image processing system 114. The imageprocessing system 114 can be configured to receive the images andgeographic location, and send the images and location or informationrelated thereto to the LiDAR control systems 110 or other systemsconnected to the LIDAR system 100.

In operation according to some examples, the LiDAR system 100 isconfigured to use nondegenerate optical sources to simultaneouslymeasure range and velocity across two dimensions. This capability allowsfor real-time, long range measurements of range, velocity, azimuth, andelevation of the surrounding environment.

In some examples, the scanning process begins with the optical drivers103 and LiDAR control systems 110. The LiDAR control systems 110instruct the optical drivers 103 to independently modulate one or moreoptical beams, and these modulated signals propagate through the passiveoptical circuit to the collimator. The collimator directs the light atthe optical scanning system that scans the environment over apreprogrammed pattern defined by the motion control system 105. Theoptical circuits 101 may also include a polarization wave plate (PWP) totransform the polarization of the light as it leaves the opticalcircuits 101. In some examples, the polarization wave plate may be aquarter-wave plate or a half-wave plate. A portion of the polarizedlight may also be reflected back to the optical circuits 101. Forexample, lensing or collimating systems used in LIDAR system 100 mayhave natural reflective properties or a reflective coating to reflect aportion of the light back to the optical circuits 101.

Optical signals reflected back from the environment pass through theoptical circuits 101 to the receivers. Because the polarization of thelight has been transformed, it may be reflected by a polarization beamsplitter along with the portion of polarized light that was reflectedback to the optical circuits 101. Accordingly, rather than returning tothe same fiber or waveguide as an optical source, the reflected light isreflected to separate optical receivers. These signals interfere withone another and generate a combined signal. Each beam signal thatreturns from the target produces a time-shifted waveform. The temporalphase difference between the two waveforms generates a beat frequencymeasured on the optical receivers (photodetectors). The combined signalcan then be reflected to the optical receivers 104.

The analog signals from the optical receivers 104 are converted todigital signals using ADCs. The digital signals are then sent to theLiDAR control systems 110. A signal processing unit 112 may then receivethe digital signals and interpret them. In some embodiments, the signalprocessing unit 112 also receives position data from the motion controlsystem 105 and galvanometers (not shown) as well as image data from theimage processing system 114. The signal processing unit 112 can thengenerate a 3D point cloud with information about range and velocity ofpoints in the environment as the optical scanner 102 scans additionalpoints. The signal processing unit 112 can also overlay a 3D point clouddata with the image data to determine velocity and distance of objectsin the surrounding area. The system also processes the satellite-basednavigation location data to provide a precise global location.

FIG. 1B is a time-frequency diagram 100 b of an FMCW scanning signal 101b that can be used by a LiDAR system, such as system 100, to scan atarget environment according to some embodiments. In one example, thescanning waveform 101 b, labeled as f_(FM)(t), is a sawtooth waveform(sawtooth “chirp”) with a chirp bandwidth Δf_(C) and a chirp periodT_(C). The slope of the sawtooth is given as k=(Δf_(C)/T_(C)). FIG. 1Balso depicts target return signal 102 b according to some embodiments.Target return signal 102 b, labeled as f_(FM)(t−Δt), is a time-delayedversion of the scanning signal 101 b, where Δt is the round trip time toand from a target illuminated by scanning signal 101 b. The round triptime is given as Δt=2R/ν, where R is the target range and ν is thevelocity of the optical beam, which is the speed of light c. The targetrange, R, can therefore be calculated as R=c(Δt/2). When the returnsignal 102 b is optically mixed with the scanning signal, a rangedependent difference frequency (“beat frequency”) Δf_(R)(t) isgenerated. The beat frequency Δf_(R)(t) is linearly related to the timedelay Δt by the slope of the sawtooth k. That is, Δf_(R)(t)=kΔt. Sincethe target range R is proportional to Δt, the target range R can becalculated as R=(c/2)(Δf_(R)(t)/k). That is, the range R is linearlyrelated to the beat frequency Δf_(R)(t). The beat frequency Δf_(R)(t)can be generated, for example, as an analog signal in optical receivers104 of system 100. The beat frequency can then be digitized by ananalog-to-digital converter (ADC), for example, in a signal conditioningunit such as signal conditioning unit 107 in LIDAR system 100. Thedigitized beat frequency signal can then be digitally processed, forexample, in a signal processing unit, such as signal processing unit 112in system 100. It should be noted that the target return signal 102 bwill, in general, also includes a frequency offset (Doppler shift) ifthe target has a velocity relative to the LIDAR system 100. The Dopplershift can be determined separately, and used to correct the frequency ofthe return signal, so the Doppler shift is not shown in FIG. 1B forsimplicity and ease of explanation. It should also be noted that thesampling frequency of the ADC will determine the highest beat frequencythat can be processed by the system without aliasing. In general, thehighest frequency that can be processed is one-half of the samplingfrequency (i.e., the “Nyquist limit”). In one example, and withoutlimitation, if the sampling frequency of the ADC is 1 gigahertz, thenthe highest beat frequency that can be processed without aliasing(Δf_(Rmax)) is 500 megahertz. This limit in turn determines the maximumrange of the system as R_(max)=(c/2)(Δf_(Rmax)/k) which can be adjustedby changing the chirp slope k. In one example, while the data samplesfrom the ADC may be continuous, the subsequent digital processingdescribed below may be partitioned into “time segments” that can beassociated with some periodicity in the LIDAR system 100. In oneexample, and without limitation, a time segment might correspond to apredetermined number of chirp periods T, or a number of full rotationsin azimuth by the optical scanner.

FIG. 2 is a block diagram illustrating an example of an opticalsubsystem of a LiDAR system 200 with a free-space polarization-diverseconfiguration according to embodiments of the present disclosure. TheLiDAR system 200 includes a photonics chip which may include a coherentLIDAR, e.g., an FMCW LIDAR, photonic integrated circuit (PIC) and/orfree-space optics. In one example, the optical circuits 101, describedwith respect to FIG. 1, may be implemented on a photonics chip. Theoptical subsystem may include the optical circuits 101, described withrespect to FIG. 1. Disclosed herein is the LiDAR system 200 with theoptical subsystem configured to efficiently collect all possiblepolarization states in a returned target signal scattered from a targetand coherently mixing the returned target signal with an appropriate LOsignal to obtain range and velocity profile of the target, e.g., byusing FMCW LiDAR architecture. A method to collect all possiblepolarization states in the returned target signal scattered from thetarget, and to coherently mix the returned target signal with theappropriate LO signal in a LiDAR system is also disclosed. Differentcombinations of polarization optics, including linear polarizers,wave-retardation optics, and magneto-optics such as Faraday rotators areutilized to achieve the above discussed objectives. The technique may beutilized in both a coaxial system and a non-coaxial system, as well asin both a free-space optics based system and a waveguide (single-mode ormulti-mode waveguide in fiber as well as semiconductor or dielectricphotonic circuit architecture) based system.

As shown in the embodiment depicted in FIG. 2, the LiDAR system 200 mayinclude an optical source 201 (e.g., a FMCW laser) which may emit anoptical beam 202. The optical beam 202 may be multi-spectral (i.e.,containing more than one wavelength). As depicted in FIG. 2, the opticalsource 201 may be optically coupled to an optical circulator 205.

The optical circulator 205 may include a first polarizing beam splittingelement (PBS) 204, a second PBS 210, a half-wave plate (HWP) 207, and anon-reciprocal optical element 208. The optical circulator 205 mayfurther include a first mirror 206 and a second mirror 209. Thenon-reciprocal optical element may include a non-reciprocal opticaldevice or a magneto-optic device. According to some embodiments, anon-reciprocal optical device can be a device through which changes inthe properties of light passing are not reversed when the light passesthrough in an opposite direction. According to one embodiment, a Faradayrotator is a non-reciprocal optical device. In such scenarios, theFaraday rotator can be used as a polarization rotator that enables thetransmission of light through a material when a longitudinal staticmagnetic field is present. In this fashion, non-reciprocal opticaldevices described herein include the functionality to rotate the stateof a polarization as a wave traverses a device. Using non-reciprocaloptical devices, as described herein, can reflect a polarized beam backthrough the same non-reciprocal optical device in a manner that does notundo the polarization change the beam underwent in its forward passthrough the medium and instead doubles the change in polarization state,compared to the initial state. In some scenarios, by implementing aFaraday rotator with a rotation of 45°, downstream reflections from alinearly polarized source will return with the polarization rotated by90°, after the reverse pass through the Faraday rotator.

Referring to FIG. 2, for instance, the non-reciprocal optical element208 may include a Faraday rotator (FR) in one embodiment. As anotherexample, the non-reciprocal optical element 208 may include an opticalelement based on an opto-mechanically induced non-reciprocity in awhispering gallery mode microcavity. In one embodiment, the beamsplitting elements may include a first polarizing beam splitter (PBS)and a second PBS. In some other embodiments, the polarizing beamsplitting elements may include a birefringent crystal or a Wollastonprism. The birefringent crystal may be a uniaxial or a biaxial crystal.Different kinds of linear optical birefringent devices may be used torealize a half-wave plate.

The optical circulator 205 may be coupled with an optical window 211that is configured to generate a local oscillator (LO) signal. Theoptical circulator 205 may have a first port 241, a second port 242 anda third port 243. The first port 241 may be coupled with the opticalsource 201. The second port 242 may be coupled with the optical window211. The third port 243 may be coupled with a PD 215. One or morescanners 213 may be coupled between the optical window 211 and a target230 to scan the target. The optical subsystem may further include one ormore lens systems 203, 212, 214 to focus or expand the optical beam 202.

The optical beam 202 may be linearly polarized at a 45° angle in the x-yplane. The optical beam 202 may have both an s-polarization componentand a p-polarization component that are in-phase. The optical beam 202may be collimated by using the lens system 203.

The optical beam 202 may pass through the PBS 204, which may separatethe s-polarization component and the p-polarization component of theoptical beam 202 into two separate paths. For example, the first PBS 204may be deposited with a coating such that the first PBS 204 may transmitthe p-polarization component of the optical beam 202 and reflects thes-polarization component of the optical beam 202. The p-polarizationcomponent of the optical beam 202 may be transmitted through the firstPBS 204 in the first path in a first direction, while the s-polarizationcomponent of the optical beam 202 may be reflected by the first PBS 204in the second path in a second direction. The s-polarization componentof the optical beam 202 may be reflected by a first mirror 206. Thefirst mirror 206 may be placed at a +45° (or −45°) with respect to anx-direction as illustrated in FIG. 2. The first PBS 204 may transmit thep-polarization component of the optical beam 202 toward the target 230in a target environment. The p-polarization component of the opticalbeam 202 is the polarization of the beam that is parallel to the planeof incidence of the optical beam 202. The first PBS 204 may reflect thes-polarization component of the optical beam 202 toward the first mirror206. The s-polarization component of the optical beam 202 is thepolarization of the beam that is perpendicular to the plane of incidenceof the optical beam 202.

Both the p-polarization component of the optical beam 202 in the firstpath and the s-polarization component of the optical beam 202 in thesecond path may pass through the HWP 207 and the non-reciprocal opticalelement 208, e.g., Faraday rotator (FR) 208, with a 45° polarizationrotation, such that both the p-polarization component and s-polarizationcomponent maintain their respective polarization.

For example, the optic axis of the HWP 207 may be arranged at 22.5° tothe x-axis, so that the s-polarization component of the optical beam 202may be rotated by +45°. The HWP 207 may introduce a polarizationrotation of +45° to the s-polarization component of the optical beam202. A thickness of the non-reciprocal optical element 208 may beselected for providing 45° polarization rotation and the rotationdirection may be selected to be counter-clockwise when light propagatesalong the z-axis in the first direction. The first direction is adirection along the z-axis as illustrated in FIG. 2. The non-reciprocaloptical element 208 may introduce a polarization rotation of −45° to thes-polarization component of the optical beam 202 in the first direction.Thus, the polarization rotation of +45° of the s-polarization componentof the optical beam 202 made by the HWP 207, which may be +45°, may becancelled by the polarization rotation of −45° of the Faraday rotator.Therefore, both the p-polarization component and s-polarizationcomponent of the optical beam 202 are unchanged after passing throughthe HWP 207 and the non-reciprocal optical element 208 in the firstdirection. The HWP 207 and the non-reciprocal optical element 208 areconfigured to maintain a polarization of the p-polarization componentand a polarization of the s-polarization component in the firstdirection. The s-polarization component of the optical beam 202 may bereflected by the second mirror 209 towards the second PBS 210. Thesecond mirror 209 may be placed at a −45° (or +45°) with respect to thex-axis.

Then, the p-polarization component of the optical beam 202 in the firstpath and the s-polarization component of the optical beam 202 in thesecond path may be combined by the second PBS 210. The second PBS 210may transmit the recombined optical beam 202 out of the opticalcirculator 205 from the second port. As shown in FIG. 2, for example,the first mirror 206 and the second mirror 209 may be placedperpendicular to each other. The first mirror 206 may be placed at a 45°angle clockwise with respect to the x-axis, and the second mirror 209may be placed at a 45° angle counterclockwise (−45°) with respect to thex-axis. The s-polarization component of the optical beam 202 and thep-polarization component of the optical beam 202 may have differentoptical path lengths in the optical circulator 205. Advantageously, thiscan be used to de-correlate the shot noise between the twopolarizations. For example, the s-polarization component of the opticalbeam 202 may have a larger optical path length than the p-polarizationcomponent of the optical beam 202 in the first direction. In otherembodiments, the optical circular may have the first mirror and thesecond mirror placed at different locations than shown in FIG. 2, whilethe s-polarization component of the optical beam and the p-polarizationcomponent of the optical beam may have a same optical path length in theoptical circulator.

The recombined optical beam 202 may pass through the optical window 211.The optical window 211 may be deposited with a special coating, suchthat optical window 211 may reflect a small first portion of the opticalbeam 202, thereby generating an LO signal 218. The optical window 211may be configured to at least partially reflect the first portion of theoptical beam 202 for generating the LO signal 218 and to transmit asecond portion of the optical beam 202 towards the target 230. Thesecond portion of the optical beam 202 may be transmitted through a lenssystem 212 and one or more scanners 213 to deliver the optical beam 202to the target 230. The lens system 212 may be configured to expand theoptical beam 202. The one or more scanners 213 may be configured to scanand illuminate the target 230 in the target environment to generate atarget return signal 216.

According to some scenarios, the target return signal 216 may bescattered or reflected from the target 230. The target return signal 216may have both an s-polarization component and a p-polarization component(with undetermined phase difference between them). The target returnsignal 216 may also have an arbitrary polarization. In some scenarios,the target return signal 216 may be collected by using the samecomponents described above as in the transmission process. After passingthrough the window 211, the target return signal 216 may co-propagatewith the LO signal 218 in a second direction opposite to the Z-axis, asillustrated in FIG. 2. Both the target return signal 216 and the LOsignal 218 may be divided into the s-polarization component and thep-polarization component in a return path by using the second PBS 210.The s-polarization component of the target return signal 216 and the LOsignal 218 may be reflected by the second mirror 209 toward thecombination of non-reciprocal optical element 208 and the HWP 207.

Next, the combination of non-reciprocal optical element 208 and the HWP207 may convert the s-polarization component in the target return signal216 and/or the LO signal 218 to a p-polarization component, and convertthe p-polarization component in the target return signal 216 and the LOsignal 218 to an s-polarization component, due to the non-reciprocalnature of the FR 208. The HWP 207 and the non-reciprocal optical element208 are configured to change a polarization of the p-polarizationcomponent and the s-polarization component of the target return signal216 and the LO signal 218 by 90° in the second direction. Due to thenon-reciprocal rotation of the FR208, in the second direction,polarization rotations by both the HWP 207 and the non-reciprocaloptical element 208 are in the same direction (not cancelled out),resulting in a total rotation of 90°.

Both the p-polarization component and the s-polarization component ofthe target return signal 216 and the LO signal 218 may pass through thefirst PBS 204 and focused and mixed on the PD 215. The p-polarizationcomponent of the target return signal 216 and the LO signal 218 may bereflected by the first mirror 206 towards the first PBS 204, and then betransmitted to the PD 215 by the first PBS 204. The s-polarizationcomponent of the target return signal 216 and the LO signal 218 may bereflected by the first PBS 204 towards the PD 215. A lens system 214 maybe used to focus the p-polarization component and the s-polarizationcomponent of the target return signal 216 and the LO signal 218 to thePD 215.

According to some embodiments, for a multi-wavelength system, the lightcan be split into the respective wavelength paths via a wavelengthdemultiplexer (DEMUX) optics such as dichroic mirror(s) (not shown) andfocused onto individual PDs for each wavelength using their respectivelens system. For example, a DEMUX optics (not shown) may be coupledbetween the third port 243 of the circulator 205 and the lens system214. The DEMUX optics may be used to separate different wavelengths anddirect them to dedicated detectors. The DEMUX optics may be for example,and without limitation, a dichroic mirror, a Bragg grating or any othersuitable wavelength demultiplexer. The separate wavelengths may then befocused by respective lens systems onto respective photodetectors.

FIG. 3 is a block diagram illustrating an example of an opticalsubsystem of a LiDAR system 300 with an alternative polarization-diverseconfiguration according to embodiments of the present disclosure. TheLiDAR system 300 is similar to the LiDAR system 200, except that a thirdPBS 321, two PDs 325, 326 and two corresponding lens systems 327, 328are employed instead of the PD 215 and the corresponding lens system 214in FIG. 2.

As shown in FIG. 3, the p-polarization component and the s-polarizationcomponent of the target return signal 216 and the LO signal 218 may besplit by using the third PBS 321. The third PBS 321 may transmit thep-polarization component of the target return signal 216 and the LOsignal 218 to a PD 325, and reflect the s-polarization component of thetarget return signal 216 and the LO signal 218 to another PD 326. Thep-polarization component and the s-polarization component of the targetreturn signal 216 may be combined with a corresponding polarizationcomponent on two separate PDs 325, 326 to generate two heterodynesignals. The lens system 327 or lens system 328 may be configured tofocus the p-polarization component or the s-polarization componentrespectively.

As illustrated in FIG. 2 and FIG. 3, the optical circulator comprisesthe first PBS 204 and the second PBS 210, the HWP 207, and thenon-reciprocal optical element 208. The optical circulator may be madeusing different methods to give the same resulting polarization states.These methods include, but are not limited to, using othernon-reciprocal or magneto-optic devices instead of the FR, usingdifferent kinds of birefringent crystals (such as uniaxial and biaxialcrystals, Wollaston prisms, etc.) instead of the PBS, and usingdifferent kinds of linear optical birefringent devices to realize awaveplate.

FIG. 4 is a block diagram illustrating an example of an opticalsubsystem of a LiDAR system 400 with another polarization-diverseconfiguration according to embodiments of the present disclosure. TheLiDAR system 400 is similar to the LiDAR system 200, except that asecond non-reciprocal optical element 410, e.g., a second FR 410,arranged to rotate a polarization by 22.5° in the first direction and torotate a polarization by 22.5° in the second direction, is coupledbetween the optical circulator 205 and the optical window 211. Thesecond non-reciprocal optical element 410 is to rotate a polarization ofthe LO signal by 45 degrees. The second non-reciprocal optical element410 may also be placed outside the optical window 211 and coupledbetween the optical window 211 and the lens system 212.

As shown in FIG. 4, the optical beam 402 may be linearly polarized andaligned to only have a p-polarization component. The FMCW laser 401 maylaunch the p-polarization component. In this case, the optical beam 402may be transmitted by the first PBS 204. After passing through thesecond PBS 210, the second non-reciprocal optical element 410 may rotatethe p-polarization component of the optical beam by 22.5°, which mayresult in the LO signal 418 to be 45° after passing through the secondnon-reciprocal optical element 410 on the return path. Since thebackscattered target return signal 416 from the target 230 may have boths-polarization and p-polarization components (with undetermined phase),having the LO signal 418 at 45° linear polarization may help in mixingthe LO signal 418 efficiently with the target return signal 416.

Similar to described in connection with FIG. 2, both the s-polarizationcomponent and the p-polarization component of the LO signal 418 and thetarget return signal 416 may be combined on the PD 215, after beingfocused by the lens system 214. Alternatively, the s-polarizationcomponent and the p-polarization component of the LO signal and thetarget return signal may be split using a third PBS, and combined on twoseparate PDs, as shown in FIG. 3.

FIG. 5 is a block diagram illustrating an example of an opticalsubsystem of a LiDAR system 500 with a polarization-diverseconfiguration according to embodiments of the present disclosure. TheLiDAR system 500 is similar to the LiDAR system 400, except that theoptical circulator 505 does not include the first mirror and the secondmirror, but includes the second non-reciprocal optical element 410, andthe optical circulator 505 has a fourth port coupled with a second PD515 and a corresponding lens system 514.

As shown in FIG. 5, the s-polarization component of the LO signal 418and the target return signal 416 may be reflected by the second PBS 210towards the lens system 514 and the second PD 515. The p-polarizationcomponent of the LO signal 418 and the target return signal 416 may becombined and mixed in the PD 215 to generate a first heterodyne signal.The s-polarization component and the p-polarization component of the LOsignal 418 and the target return signal 416 may be combined and mixed inthe second PD 515 to generate a second heterodyne signal.

FIG. 6 is a block diagram illustrating an example of an opticalsubsystem of a LiDAR system 600 according to embodiments of the presentdisclosure. The FMCW laser 401 may launch the p-polarization componentof the optical beam 402. A target 630 may be known to primarily scatteran incident light in the same polarization as an input linearpolarization of the incident light, which may be true for most diffusescattered objects that are of interest in an automotive LIDAR system. Inthis case, the PBS 240 and the non-reciprocal optical element 208 may beused to direct the optical beam 402 instead of the optical circulator205 or the optical circulator 505.

As shown in FIG. 6, the p-polarization component of the optical beam 402is output from the FMCW laser 401, and pass through a transmit port ofthe PBS 204. The non-reciprocal optical element 208 with the 45°polarization rotation is used to rotate the p-polarization component toa linear combination of an s-polarization component and a p-polarizationcomponent (in-phase). After passing through the LO-generating window211, the beam-expanding lens system 212 and the one or more scanners213, the linear combination of the s-polarization component and thep-polarization component impinge on the target 630. The target 630 mayreflect light primarily in the same polarization as the input linearcombination. A target return signal 616 may be received by the sameoptics as the transmitted beam. The target return signal 616 may havethe same linear combination of the s-polarization component and thep-polarization component as the incident beam. The LO signal 618,generated by a window plate, has the same polarization (45°, or acombination of s-polarization and p-polarization component, in-phase).The LO signal 618 along with the co-axial target return signal 616 maybe converted to an s-polarization component after passing through thenon-reciprocal optical element 208 on a return path. The PBS 204 maythen reflect both the LO signal 618 and the target return signal 616.Both the LO signal 618 and the target return signal 616 may be focusedusing the lens system 214 and mixed on the PD 215.

FIG. 7 is a block diagram illustrating an example of an opticalsubsystem of a LiDAR system 700 according to embodiments of the presentdisclosure. A fiber-based architecture (single-mode or multi-mode,including polarization-maintaining) may be used, as shown in FIG. 7. TheFWCW laser may emit an optical beam 702 with a p-polarization component.An LO signal 718 may be generated using a beam splitter 711 (based on afused fiber coupler or any other technology). A beam combiner 740 may beused to combine and mix the LO signal 718 and a target return signal 716on the PD 715. A balanced, differential or any other suitable detectionmode of the PD may be used.

Here, a fiber-version FR (not shown) may be used, or an externalfree-space FR 708 may also be used, before or after the lens system 712.As discussed above, the target return signal 716 may have ans-polarization component after passing the PBS 704 on a return pass. Afiber rod rotation or a cross-polarization splice (made by splicing twopolarization-maintaining fibers such that the slow axis of the twofibers are orthogonally aligned) or an appropriate waveplate may be usedto convert a polarization from s top or vice-versa, in either the LO ortarget fiber path. As shown in FIG. 7, a cross-polarization splice 750is used to convert the s-polarization component of the target returnsignal 716 into a p-polarization component in the target return path.

FIG. 8 is a flow diagram illustrating an example method 800 in a LiDARsystem, according to embodiments of the present disclosure. Variousportions of method 800 may be performed by LiDAR system 200, 300, 400,500, 600 and 700, as illustrated in FIGS. 2, 3, 4, 5, 6 and 7respectively and described in detail above.

With reference to FIG. 8, method 800 illustrates example functions usedby various embodiments. Although specific function blocks (oroperations) are disclosed in method 800, such blocks (or operations) areexamples. That is, embodiments are well suited to performing variousother blocks (or operations) or variations of the blocks (or operations)recited in method 800. It is appreciated that the blocks (or operations)in method 800 may be performed in an order different than presented, andthat not all of the blocks (or operations) in method 800 may beperformed.

At 802, an optical source of a LiDAR system generates an optical beam.The optical sources may generate multiple beams, and each beam may havemultiple wavelengths. For example, the optical beam may be linearlypolarized at a 45° angle in the x-y plane. For another example, theoptical beam may be p-polarization or s-polarization with respect to aPBS that receives the optical beam.

At 804, the optical beam is received, by an optical circulator.

At 806, the optical circulator transmits the optical beam to a target.

At 808, the optical circulator receives a target return signal returnedfrom the target, including the optical circulator collects allpolarization states in the target return signal.

At 810, the optical circulator transmits the target return signal to oneor more photodetectors (PDs).

At 812, an optical component generates an LO signal.

At 814, the one or more PDs mix the target return signal with the LOsignal to generate a heterodyne signal to extract range and velocityinformation of the target.

The preceding description sets forth numerous specific details such asexamples of specific systems, components, methods, and so forth, inorder to provide a thorough understanding of several examples in thepresent disclosure. It will be apparent to one skilled in the art,however, that at least some examples of the present disclosure may bepracticed without these specific details. In other instances, well-knowncomponents or methods are not described in detail or are presented insimple block diagram form in order to avoid unnecessarily obscuring thepresent disclosure. Thus, the specific details set forth are merelyexemplary. Particular examples may vary from these exemplary details andstill be contemplated to be within the scope of the present disclosure.

Any reference throughout this specification to “one example” or “anexample” means that a particular feature, structure, or characteristicdescribed in connection with the examples are included in at least oneexample. Therefore, the appearances of the phrase “in one example” or“in an example” in various places throughout this specification are notnecessarily all referring to the same example.

Although the operations of the methods herein are shown and described ina particular order, the order of the operations of each method may bealtered so that certain operations may be performed in an inverse orderor so that certain operation may be performed, at least in part,concurrently with other operations. Instructions or sub-operations ofdistinct operations may be performed in an intermittent or alternatingmanner.

The above description of illustrated implementations of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific implementations of, and examples for, the invention aredescribed herein for illustrative purposes, various equivalentmodifications are possible within the scope of the invention, as thoseskilled in the relevant art will recognize. The words “example” or“exemplary” are used herein to mean serving as an example, instance, orillustration. Any aspect or design described herein as “example” or“exemplary” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Rather, use of the words“example” or “exemplary” is intended to present concepts in a concretefashion. As used in this application, the term “or” is intended to meanan inclusive “or” rather than an exclusive “or”. That is, unlessspecified otherwise, or clear from context, “X includes A or B” isintended to mean any of the natural inclusive permutations. That is, ifX includes A; X includes B; or X includes both A and B, then “X includesA or B” is satisfied under any of the foregoing instances. In addition,the articles “a” and “an” as used in this application and the appendedclaims should generally be construed to mean “one or more” unlessspecified otherwise or clear from context to be directed to a singularform. Furthermore, the terms “first,” “second,” “third,” “fourth,” etc.as used herein are meant as labels to distinguish among differentelements and may not necessarily have an ordinal meaning according totheir numerical designation.

What is claimed is:
 1. A light detection and ranging (LiDAR) system,comprising: one or more photodetectors (PDs); and an optical circulatorto receive an optical beam and transmit the optical beam to a target, toreceive a target return signal from the target and to transmit thetarget return signal to the one or more PDs, wherein the one or more PDsare to mix the target return signal with a local oscillator (LO) signalto generate a heterodyne signal to extract range and velocityinformation of the target.
 2. The LiDAR system of claim 1, wherein theoptical circulator comprises a first polarizing beam splitting elementand a non-reciprocal optical element.
 3. The LiDAR system of claim 2,wherein the non-reciprocal optical element comprises a Faraday rotator(FR).
 4. The LiDAR system of claim 2, wherein the first polarizing beamsplitting element includes a first polarizing beam splitter (PBS). 5.The LiDAR system of claim 2, wherein the optical circulator furthercomprises a half-wave plate and a second polarizing beam splittingelement.
 6. The LiDAR system of claim 1, further comprising an opticalwindow to generate the LO signal, and wherein the optical circulator isfurther to receive the LO signal returned from the optical window, andto transmit the LO signal to the one or more PDs.
 7. The LiDAR system ofclaim 6, further comprising a second non-reciprocal optical elementdisposed between the optical circulator and the optical window, whereinthe second non-reciprocal optical element is to rotate a polarization ofthe LO signal by 45 degrees.
 8. The LiDAR system of claim 1, wherein theoptical source comprises a frequency modulated continuous-wave (FMCW)laser.
 9. The LiDAR system of claim 1, further comprising a beamsplitter to generate the LO signal, and wherein the one or more PDs areto receive the LO signal generated by the beam splitter directly. 10.The LiDAR system of claim 1, further comprising a cross-polarizationsplice, wherein the cross-polarization splice is to convert ans-polarization component of the target return signal into ap-polarization component in a target return path.
 11. The LiDAR systemof claim 1, further comprising a beam combiner to combine and mix the LOsignal and the target return signal on the one or more PDs.
 12. A methodof light detection and ranging (LiDAR), comprising: transmitting, by anoptical circulator, an optical beam to a target; receiving, by theoptical circulator, a target return signal returned from the target;transmitting, by the optical circulator, the target return signal to oneor more photodetectors (PDs); and mixing, by the one or more PDs, thetarget return signal with a local oscillator (LO) signal to generate aheterodyne signal to extract range and velocity information of thetarget.
 13. The method of claim 12, wherein the optical circulatorcomprises a first polarizing beam splitting element and a non-reciprocaloptical element.
 14. The method of claim 13, wherein the non-reciprocaloptical element comprises a Faraday rotator (FR).
 15. The method ofclaim 13, wherein the first polarizing beam splitting element comprisesa first polarizing beam splitter (PBS).
 16. The method of claim 13,wherein the optical circulator further comprises a half-wave plate and asecond polarizing beam splitting element.
 17. The method of claim 13,further comprising rotating, by a second non-reciprocal optical element,a polarization of the LO signal by 45 degrees.
 18. The method of claim12, further comprising: generating the LO signal by an optical window;receiving the LO signal, by the optical circulator, returned from theoptical window; and transmitting the LO signal, by the opticalcirculator, to the one or more PDs.
 19. The method of claim 12, furthercomprising: generating the LO signal by a beam splitter; and receivingthe LO signal by the one or more PDs directly.
 20. The method of claim12, further comprising scanning, by one or more scanners, the target togenerate the target return signal.